With recent technological developments, optical fibers can now move beyond the realm of voice and data transport to function as a sensor for measuring a variety of environmental conditions in a diverse number of applications, and harsh conditions, where conventional measurement systems are not practical.
Previously, discrete methods have been used to gather environmental data. These discrete methods use single-point sensors such as thermometers, resistance strain gauges, piezoelectric sensors, acoustic sensors, and the like (strategically, but discretely, located on or within the structure, thus leaving large areas or portions not covered) to monitor environment situations associated with structures. These structures may include, for example, bridges and buildings where there is a need to monitor structural integrity. Other structures include oil and gas wells, where downhole temperature and pressure readings are used to ensure that the well is operating properly. If these discrete, single-point sensors are not located within the region(s) of the structure where a problem arises, accurate and timely detection of an emergency condition cannot be ensured.
“Distributed” optical fiber-based sensors have been developed that are disposed alongside a structure (or within a borehole for well monitoring purposes), with changes in the characteristics of the light propagating along the fiber caused by the changes in its environment, analyzed to detect a variety of different problems by indicating changes in temperature, pressure, strain, etc. In any case, the distributed strain sensing (DSS) fiber-based cable must be sufficiently robust to withstand installation and handling, as well as any extreme environmental conditions after installation, while efficiently and accurately transferring the load stress due to dimensional changes of deformation of the sensing optical fiber. The cable sensor must not impart any stress related to installation, handling and/or extreme environmental conditions onto the sensing optical fiber, or the strain sensing characteristics of the sensor will be compromised.
Additionally, it is preferred that the cable itself retain no mechanical “memory” after bending, which may easily occur during installation. Such memory results in inaccurate strain and perhaps hysteresis in measurements. For the DSS cable to function and provide accurate, uniform and continuous strain measurements, there must be sufficient coupling of the sensing optical fiber to the cable jacket and the sensing cable strength elements.